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The Porous PavementCurve Number

Thomas Ballestero, PE, PhD, PH, CGWP, PG, Federico Uribe,Robert Roseen, PE, PhD, D.WRE, James Houle, CPSWQ

University of New Hampshire Stormwater CenterUniversity of New Hampshire

Philadelphia Low Impact Development Symposium: Greening the Urban Environment25-28 September 2011

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What is the Curve Number For Porous Pavement?

Who wants to know?!?

(What is your OBJECTIVE?)

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The SCS (NRCS) Curve NumberOriginally conceived to translate rainfall depth into runoff depth on agricultural watersheds…method worked best for large storms

This was then translated into a runoff hydrograph

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Definition Sketch of SCS Runoff Hydrograph Characteristics

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5

UNHSC Porous Pavement Sites

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UNHSC Porous Asphalt Lot UNHSC Porous Concrete Lot7

Permeable Pavement Sites

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Typical Cross‐Section Construction

SUBGRADE

NATIVE MATERIALS

BANK RUN GRAVEL FILTER COURSE

PERVIOUS PAVEMENT 3-6”

1-1/4” CRUSHED STONE CHOKER COURSE

14”

3/8” PEA-GRAVEL RESERVOIR COURSE 6”4”

Sub-base design matches that of the UNHSC Porous Asphalt Parking Lot

4”

UNHSC Porous Pavement Monitoring

Compound weir Pressure transducer Datalogger

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10

REFERENCE LOT

POROUS ASPHALT

Tree Filter

UNHSC Porous Pavement Hydrologic Data

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“Real time” flow monitoring…5‐minute time step “Real time” rainfall monitoring…5‐minute time step

0

200

400

600

800

1000

1200

1400

1/1/08

1/8/08

1/15/0

8

1/22/0

8

1/29/0

8

2/5/08

2/12/0

8

2/19/0

8

2/26/0

8

3/4/08

3/11/0

8

3/18/0

8

3/25/0

8

Volu

me

(gal

.)

Influent

Effluent

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0

200

400

600

800

1000

1200

1400

1600

Volu

me

(gal

.)

Influent

Effluent

PC Flow Attenuation1/1/08 - 3/31/08

Influent Effluent

Total Volume (liters) 446,034 78,192

# of Flow Events 16 8

4/1/08 - 6/30/08

Influent Effluent

Total Volume (liters) 446,034 25,585

# of Flow Events 15 5

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PC Pollutant Removal

82% RE94% RE

Methods of Teasing CN from the Data

Measure P and Q, invert basic SCS equation Measure P and outflow hydrograph (q), measure lag, 

estimate CN from lag equations Measure Q and qp, estimate CN from peak discharge 

equations

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Q P Ia 2

P Ia SEq. 1.

S 1000CN

10 Eq. 4.

Ia 0.2S Eq. 2.

.3.8.02.0

8.0

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EqSPSP

SPIPQ a

Q: Total Runoff Depth (in)P: Total Precipitation Depth (in)Ia: Initial Abstraction (in)S: Storage Parameter (in)

Method 1 ‐ Depth of Runoff (Q)MethodMethod 1 ‐ Depth of Runoff (Q)Method

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Study how the timing of the “runoff” is transformed

Time of concentration Lag time Time base Peak time

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Method 2 ‐Lag MethodsMethod 2 ‐Lag Methods

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Tlag: Lag Time (hr)Tc: Concentration Time (hr)Y: Surface Slope (%)S: Storage Parameter (in)

Tlag L0.8 S 1 0.7

1900Y 0.5 Eq. 5.

Tc 53

Tlag Eq. 6.

S 1000CN

10 Eq. 7.

Method 2 ‐LagMethods

Method 2 ‐LagMethods

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0

1

2

3

4

5

6

0

0.01

0.02

0.03

0.04

0.05

0.06

0 50 100 150 200 250 300 350 400 450 500 550 600

Run

off

[gpm

]

Rai

nfal

l [in

]

Lag MethodsLag Methods

1. T base (Sánchez San Román [2009])

T base = T precip + T conc

T conc = T base – T precip

LAG METHOD (A) – lag measured from precip peak and runoff peak

3 APPROACHES

T baseT 

precip

Using Eq. 5 and Eq. 6 used into Eq.7., solve for CN

In. Abs.

Recession curveRecession curve

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0

1

2

3

4

5

6

0

0.01

0.02

0.03

0.04

0.05

0.06

0 50 100 150 200 250 300 350 400 450 500 550 600

Run

off

[gpm

]

Rai

nfal

l [in

]

2. T peak (Sánchez San Román [2009], Folmar, Miller and Woodward [2007])

T peak = T lag + T precip/2

T lag = T peak– T precip/2

LAG METHOD (B) – measure lag from duration of excessprecipitation

T peak

T precip

Insert Eq. 1 into Eq.3 and solve for CN:

In. Abs.

T lag

CN 1000

1900 TLAG Y 0.5

L0.8

1.423

9

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Lag MethodsLag Methods

0

1

2

3

4

5

6

0

0.01

0.02

0.03

0.04

0.05

0.06

0 50 100 150 200 250 300 350 400 450 500 550 600

Run

off

[gpm

]

Rai

nfal

l [in

]

3. T centroid (NRCS [2009], Folmar, Miller and Woodward [2009])

T lag:  time from the centroid of excess precipitation to the peak of the hydrograph. 

LAG METHOD (C) – Measure Lag from when ½ Q occurs

In. Abs.

T lag

CN 1000

1900 TLAG Y 0.5

L0.8

1.423

9

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Lag MethodsLag Methods

qp: Peak Discharge (cfs)qu: Unit Peak Discharge (csm/in) Am: Drainage area (mi2) Q: Runoff (in)

Method 3GRAPHICAL PEAK DISCHARGE METHOD

Method 3GRAPHICAL PEAK DISCHARGE METHOD

0

1

2

3

4

5

6

0 50 100 150 200 250 300 350 400 450 500 550 600

Run

off [

gpm

]

qp qu Am Q

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qp qu Am Q Eq. 8.

GRAPHICAL PEAK DISCHARGE METHODGRAPHICAL PEAK DISCHARGE METHOD

0

1

2

3

4

5

6

0 50 100 150 200 250 300 350 400 450 500 550 600

Run

off [

gpm

]

qu qp

Q Am

Eq. 9.

Area = 0.000201 mi2(5200 ft2)

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Method 3 – GraphicalPeak Discharge

Method 3 – GraphicalPeak Discharge

Tc estimated with Lag Method and qufound with Eq. 9., find Ia/P from the Unit Peak discharge for NRCS type III 

rainfall distribution chart.

★

If Ia/P < 0.1 then Ia/P=0.1If Ia/P > 0.5 then Ia/P=0.5

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Method 3 – GraphicalPeak Discharge

Method 3 – GraphicalPeak Discharge

Knowing Ia/P and P, compute Ia. Then with Eq. 2 and Eq. 7, 

obtain CN

S 1000CN

10 Eq. 7.

CN 1000

5 Ia 10

Ia 0.2S Eq. 2.

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CNMethod 1

CNMethod 2Method A

CNMethod 2Method B

CNMethod 2Method C

CNMethod 3

Average 74 11 6 6 51Median 75 8 2 3 13

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RESULTSRESULTS

Natural state for Hinckley-Charlton soil (HSG – B/C)= 60 - 72

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So….Which to Use? Events

Peak Outflow from Underdrain Peak flow method

No net increase in benchmark storms Lag method (median)

Long Term Simulation Lag methods Runoff depth method (~native soil)

Watershed Simulation Seasonal CN Lag methods

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Philosophically Speaking….. What is the CN for a detention pond?

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time

flowwatershed hydrograph

pond hydrograph

REFERENCESREFERENCES

FOLMAR,N.D; MILLER, A.C.; AND WOODWARD, W.E; 2007. “HISTORY AND DEVELOPMENT OF THE NRSC LAG TIME EQUATION”. JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION. VOL. 43(3): 829‐838. 

LEMAY, G; 2008. DETERMINING THE CURVE NUMBER (CN) FOR POROUS ASPHALT SYSTEMS –INDIVIDUAL STORM VOLUMES. HONORS THESIS INDEPENDENT

SANCHEZ SAN ROMAN, F.J. 2009. “HIDROLOGIA SUPERFICIAL III”. ONLINE HTTP://WEB.USAL.ES/JAVISAN/HIDRO

UNITED STATES DEPARTMENT OF AGRICULTURE. NATIONAL RESOURCES CONVERSATION SERVICE. 1986. “URBAN HYDROLOGY FOR SMALL WATERSHEDS TR‐55”. ONLINE HTTP://WWW.WSI.NRCS.USDA.GOV/PRODUCTS/W2Q/H&H/TOOLS_MODELS/OTHER/TR55.HTML

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AcknowledgementsFunding Source:

Questions?

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http://www.unh.edu/erg/cstev/

or Simply Search for “UNHSC”